Exhaust gas purification device of internal combustion engine

ABSTRACT

A catalyst ( 22 ) suitable for reduction of the NO x  in an exhaust gas by ammonia in the presence of excess oxygen is arranged in the exhaust passage ( 18, 21 ) of an internal combustion engine. An aqueous urea solution is fed through a flow control valve ( 33 ) to the inside of the exhaust passage ( 21 ) upstream of the catalyst ( 22 ). When the temperature of the catalyst ( 22 ) is low, a large amount of the aqueous urea solution is fed to make the urea contained in the aqueous urea solution be stored in the catalyst ( 22 ). When the engine is accelerated and the temperature of the catalyst ( 22 ) rises, ammonia is released at a little at a time from the inside of the catalyst ( 22 ) and the NO x  in the exhaust gas is reduced by the released ammonia.

This is a continuation of application Ser. No. 10/943,946 filed 20 Sep.2004 now U.S. Pat. No. 6,959,540, which is a continuation of applicationSer. No. 09/720,489 filed 22 Dec. 2000 now abandoned, which is a 371 ofPCT/JP99/03349 filed 23 Jun. 1999 and which claims priority to JapanesePatent Application No. 10-175637 filed 23 Jun. 1998, the contents ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification device ofan internal combustion engine.

BACKGROUND ART

It is possible to place a catalyst suitable for the reduction of NO_(x)in an exhaust gas by ammonia and to feed an aqueous urea solution intothe engine exhaust passage upstream of the catalyst to cause reductionof the NO_(x) in exhaust gas by the ammonia generated from the aqueousurea solution. In this case, however, the purification rate of theNO_(x) becomes lower along with a fall in the temperature of thecatalyst. Therefore, known in the art has been an internal combustionengine designed to calculate the equivalent ratio of the urea necessaryfor reducing NO_(x) by an NO_(x) purification rate in accordance withthe catalyst temperature and to control the amount of feed of theaqueous urea solution so that urea is fed by that equivalent ratio (seeJapanese Unexamined Utility Model Publication (Kokai) No. 3-129712).

So long as feeding urea by an equivalent ratio calculated in this way,however, when the catalyst temperature is not that high, the NO_(x)purification rate is low. Therefore, when the catalyst temperature isnot that high, there is the problem that a high NO_(x) purification ratecannot be obtained. In particular, when the catalyst temperature doesnot become that high and the amount of NO_(x) in the exhaust gas islarge, such as when the engine is accelerating from a low load operatingstate, if the NO_(x) purification rate is low, the problem will arise ofa large amount of NO_(x) being released into the atmosphere.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust gaspurification device of an internal combustion engine enabling the NO_(x)purification rate to be made higher.

According to the present invention, there is provided an exhaust gaspurification device of an internal combustion engine arranging acatalyst suitable for reduction of NO_(x) in exhaust gas by ammonia inthe present of excess oxygen in an engine exhaust passage, the devicecomprising feeding means for feeding a liquid containing an ammoniagenerating compound to the catalyst and feed control means forcontrolling the amount of feed of the liquid, the catalyst storinginside it at least part of the ammonia generating compound contained inthe liquid fed to the catalyst and having the function of causing therelease of ammonia a little at a time from the ammonia generatingcompound stored in the catalyst along with the rise of temperature ofthe catalyst and reducing the NO_(x) in the exhaust gas by the ammoniareleased, the device further comprising decision means for deciding ifthe temperature of the catalyst is in an ammonia generating compoundstorage region which stores the ammonia generating compound contained inthe liquid fed to the catalyst inside the catalyst and does not releasealmost any ammonia from the stored ammonia generating compound or is inan ammonia release region which causes the ammonia to be released alittle at a time from the stored ammonia generating compound, the feedcontrol means feeding a sufficient amount of the liquid to the catalystwhen the temperature of the catalyst is in the ammonia generatingcompound storage region, said sufficient amount of the liquid beingnecessary to store in the catalyst in advance enough of the ammoniagenerating compound for releasing the amount of ammonia required whenthe temperature of the catalyst becomes the ammonia release region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of an internal combustion engine,

FIG. 2 is a view of the NO_(x) purification rate,

FIG. 3 is a view of the concentration of ammonia generated,

FIG. 4 is a view of the temperature of the catalyst and theconcentration of ammonia generated,

FIG. 5 is a time chart of the NO_(x) reduction processing,

FIGS. 6A, 6B, and 6C are views of the equivalent ratio of urea requiredfor reducing NO_(x) in exhaust gas,

FIG. 7 is a view of the ratio of storage of urea,

FIGS. 8A, 8B, and 8C and

FIGS. 9A, 9B, and 9C are views of the ratio of release of ammonia,

FIG. 10 to FIG. 12 are flow charts of the control of the feed of theaqueous urea solution,

FIG. 13 is a view of the ratio of increase of urea,

FIG. 14 is a time chart of the NO_(x) reduction processing,

FIG. 15 is a flow chart for the execution of a feed control I,

FIG. 16 is a time chart of the NO_(x) reduction processing,

FIG. 17 is a flow chart for the execution of a feed control II,

FIGS. 18A, 18B, 18C, 18D, and 18E and

FIGS. 19A and 19B are views of various examples of catalysts, and

FIG. 20 to FIG. 23 are overviews of internal combustion engines showingvarious embodiments.

BEST MODE FOR WORKING THE INVENTION

FIG. 1 shows the case of application of the present invention to acompression ignition type internal combustion engine. Note that thepresent invention can also be applied to a gasoline engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a cylinder block, 3 acylinder head, 4 a piston, 5 a combustion chamber, 6 an electricallycontrolled fuel injector, 7 an intake valve, 8 an intake port, 9 anexhaust valve, and 10 an exhaust port. The intake port 8 is connected toa surge tank 12 through a corresponding intake pipe 11, while the surgetank 12 is connected to an air cleaner 15 through an intake duct 13 andan air flow meter 14. Inside the intake duct 13 is arranged a throttlevalve 17 driven by a step motor 16.

On the other hand, an exhaust port 10 is connected to an inlet of afirst catalytic converter 20 housing a catalyst 19 through an exhaustmanifold 18. The outlet of the first catalytic converter 20 is connectedto a second catalytic converter 23 housing a catalyst 22 through anexhaust pipe 21. In the embodiment shown in FIG. 1, the catalyst 19 iscomprised of a catalyst having an oxidation function, for example, anoxidation catalyst or a three-way catalyst, while the catalyst 22 iscomprised of an NO_(x) selective reduction catalyst suitable forreduction of NO_(x) in exhaust gas by ammonia in the presence of anexcess of oxygen.

The exhaust manifold 18 and the surge tank 12 are connected to eachother through an exhaust gas recirculation (hereinafter referred to asEGR) passage 24. Inside the EGR passage 24 is arranged an electricallycontrolled EGR control valve 25. The fuel injectors 6 are connected to afuel reservoir, a so-called common rail 27, through fuel feed pipes 26.Fuel is fed into the common rail 27 from an electrically controlledvariable discharge fuel pump 28. The fuel fed into the common rail 27 isfed to the fuel injectors 6 through the fuel feed pipes 26. The commonrail 29 has a fuel pressure sensor 29 attached to it for detecting thefuel pressure in the common rail 27. The discharge of the fuel pump 28is controlled based on the output signal of the fuel pressure sensor 29so that the fuel pressure in the common rail 27 becomes a target fuelpressure.

On the other hand, a liquid containing an ammonia generating compoundfor generating ammonia is stored in a tank 30. The liquid containing theammonia generating compound stored in the tank 30 is fed into theexhaust pipe 21 through a feed conduit 31, a feed pump 32, and anelectromagnetically controlled flow control valve 33.

An electronic control unit 40 is comprised of a digital computerprovided with a ROM (read only memory) 42, RAM (random access memory)43, CPU (microprocessor) 44, input port 45, and output port 46 connectedto each other through a bidirectional bus 41. The air flow meter 14generates an output voltage proportional to the amount of intake air.This output voltage is input through a corresponding AD converter 47 tothe input port 45. Further, the output signal of the fuel pressuresensor 29 is input through a corresponding AD converter 47 to the inputport 45. On the other hand, the engine body 1 has attached to it a watertemperature sensor 34 for detecting the engine coolant watertemperature, while the exhaust pipe 21 has arranged in it a temperaturesensor 35 for detecting the temperature of the exhaust gas flowingthrough the exhaust pipe 21. The output signals of these watertemperature sensor 34 and temperature sensor 35 are input to the inputport 45 through the corresponding AD converters 47.

An accelerator pedal 50 has connected to it a load sensor 51 generatingan output voltage proportional to the amount of depression L of theaccelerator pedal 50. The output voltage of the load sensor 51 is inputto the input port 45 through the corresponding AD converter 47. Further,the input port 45 has connected to it a crank angle sensor 52 generatingan output pulse each time a crankshaft rotates by for example 30degrees. Further, an actuation signal of a starter switch 53 is input tothe input port 45. On the other hand, the output port 46 is connectedthrough corresponding drive circuits 54 to the fuel injectors 6, thestep motor 16, the EGR control valve 25, the fuel pump 28, the pump 32,and the flow control valve 33.

Further, as explained above, a liquid containing an ammonia generatingcompound is fed into the exhaust pipe 21 upstream of the catalyst 22.Regarding the ammonia generating compounds able to generate ammonia,there are various such compounds. Therefore, it is possible to usevarious compounds as the ammonia generating compound. In this embodimentof the present invention, urea is used as the ammonia generatingcompound, while an aqueous urea solution is used as the liquidcontaining an ammonia generating compound. Therefore, below, the presentinvention will be explained taking as an example the case of feeding anaqueous urea solution into the exhaust pipe 21 upstream of the catalyst22.

On the other hand, as explained above, the catalyst 22 is comprised ofan NO_(x) selective reduction catalyst. In the embodiment shown in FIG.1, titania is used as the carrier of this NO_(x) selective reductioncatalyst. A catalyst V₂O₅/TiO₂ comprising vanadium oxide carried on thiscarrier (hereinafter referred to as a vanadium titania catalyst) or acatalyst Cu/ZSM 5 comprising zeolite as a carrier and copper carried onthis carrier (hereinafter referred to as a copper zeolite catalyst) isused.

When the aqueous urea solution is fed into exhaust gas containing anexcess of oxygen, the NO contained in the exhaust gas is reduced by theammonia NH₃ generated from the urea CO(NH₂)₂ on the catalyst 22 (forexample, 2NH₃+2NO+½O[²]₂2N₂+3H₂O). In this case, a certain amount ofurea is required to reduce the NO_(x) contained in the exhaust gas andcompletely remove the NO_(x) in the exhaust gas. Below, the amount ofurea required for reducing and completely removing the NO_(x) in theexhaust gas will be called the equivalent ratio of urea.

FIG. 2 shows the NO_(x) purification rate in the case of feeding anaqueous urea solution so that the amount of urea becomes the equivalentratio for the amount of NO_(x) in the exhaust gas when changing thetemperature Ti of the exhaust gas flowing into the catalyst 22 whilemaintaining a constant engine speed. Note that the solid line in FIG. 2shows the case of use of a copper zeolite catalyst as the catalyst 22,while the broken line shows the case of use of a vanadium titaniacatalyst as the catalyst 22.

From FIG. 2, it is understood that when aqueous urea solution is fed sothat the amount of urea becomes the equivalent ratio with respect to theamount of NO_(x) in the exhaust gas, for all catalysts 22, when thetemperature Ti of the exhaust gas flowing into the catalyst 22 becomesabout 350° C. or more, the NO_(x) purification rate becomes about 100percent and that as the temperature Ti of the exhaust gas flowing intothe catalyst 22 falls, the NO_(x) purification rate falls.

On the other hand, FIG. 3 shows the relation between the elapsed time t(sec) from the feeding of the aqueous urea solution and theconcentration of the ammonia generated (ppm) when feeding an aqueousurea solution in the state with the temperature of the catalyst 22maintained at 400° C. From FIG. 3, it is understood that when feedingthe aqueous urea solution, the urea is decomposed into ammonia all atonce and ammonia is released all at once. Further, as explained above,when the temperature of the catalyst 22 is 400° C., if the urea is fedby the equivalent ratio, the NO_(x) purification rate becomessubstantially 100 percent.

Therefore, from FIG. 2 and FIG. 3, when the temperature of the catalyst22 is over about 350° C., if an aqueous urea solution is fed so that theamount of urea becomes the equivalent ratio for the amount of NO_(x) inthe exhaust gas, ammonia is released all at once from the urea containedin the aqueous urea solution and all of the NO_(x) in the exhaust gascan be reduced by the ammonia. In other words, when the temperature ofthe catalyst 22 is more than about 350° C., if an aqueous urea solutionis fed so that the amount of urea becomes the equivalent ratio for theamount of NO_(x) in the exhaust gas, it becomes possible tosubstantially completely purify the NO_(x) in the exhaust gas.

On the other hand, FIG. 4 shows the relationship between the elapsedtime t (sec) from the start of feeding of the aqueous urea solution andthe concentration of the ammonia generated (ppm) when feeding an aqueousurea solution when the temperature Tc of the catalyst 22 is 120° C. andthen gradually raising the temperature Tc of the catalyst 22. As shownin FIG. 4, even if the aqueous urea solution is fed, no ammonia at allis generated while the temperature Tc of the catalyst 22 is low. Whenthe temperature Tc of the catalyst 22 starts to rise, ammonia isgenerated a little at a time as the temperature Tc of the catalyst 22rises.

FIG. 4 means the following two facts: That is, first, the fact thatammonia is generated when the temperature Tc of the catalyst 22 risesmeans that the fed urea is stored in the catalyst 22. Second, thetemperature of heat decomposition of urea is about 132° C., therefore,considering the fact that ammonia is generated by the heat decompositionof urea, ammonia should be released all at once when the temperature Tcof the catalyst 22 reaches about 132° C. As shown in FIG. 4, however,even if the temperature Tc of the catalyst 22 reaches about 132° C.,ammonia is not released all at once. This means that ammonia is notbeing generated by just the heat decomposition of the urea.

The fact that even if the temperature Tc of the catalyst 22 reachesabout 132° C., the ammonia is not released all at once and ammonia isreleased a little at a time as the temperature Tc of the catalyst 22rises is believed to be due to the morphological change of the urea onthe catalyst 22. That is, urea changes to biuret at about 132° C.,biuret changes to cyanuric acid at about 190° C., and cyanuric acidchanges to cyanic acid or isocyanic acid at about 360° C. It is believedthat ammonia is generated a little at a time in the process of thesemorphological changes due to the rising temperature. Therefore, as shownin FIG. 4, ammonia is released a little at a time from the catalyst 22as the temperature Tc of the catalyst 22 rises.

That is, if the temperature Tc of the catalyst 22 is low when feedingthe aqueous urea solution, the urea contained in the aqueous ureasolution is stored in the catalyst 22. Next, if the temperature Tc ofthe catalyst 22 rises, the urea stored in the catalyst 22 successivelychanges in form to different ammonia generating compounds along withthis. As a result, ammonia is gradually released from the catalyst 22.

In this way, if the temperature Tc of the catalyst 22 is low whenfeeding the aqueous urea solution, the urea contained in the aqueousurea solution is stored in the catalyst 22. So long as the temperatureTc of the catalyst 22 is maintained in the low state, the urea continuesto be stored in the catalyst 22. On the other hand, if the aqueous ureasolution is fed when the temperature Tc of the catalyst 22 is betweenabout 132° C. and about 350° C., the urea contained in the aqueous ureasolution is stored once in the catalyst 22 also. Next, when thetemperature of the urea rises and the urea successively changes in formto different ammonia generating compounds, ammonia is released from thecatalyst 22. That is, if the aqueous urea solution is fed while thetemperature Tc of the catalyst 22 is between about 132° C. to 350° C.,the action of release of ammonia from the catalyst 22 is started alittle while after that.

In this way, if the aqueous urea solution is fed when the temperature Tcof the catalyst 22 is between about 132° C. and 350° C., the action ofrelease of the ammonia from the catalyst 22 starts in a short whileafter that. When the temperature Tc of the catalyst 22 is maintainedsubstantially constant, if the aqueous urea solution continues to befed, ammonia continues to be released from the catalyst 22. In thiscase, however, the urea stored in the catalyst 22 only changes in formto the ammonia generating compound determined by the temperature Tc ofthe catalyst 22, so not that much ammonia is generated. Accordingly, inthis case, even if aqueous urea solution is fed so that the amount ofurea becomes the equivalent ratio for the amount of NO_(x) in theexhaust gas, all of the NO_(x) in the exhaust gas will not be completelyreduced by the ammonia generated from the catalyst 22.

Further, it is considered that when the aqueous urea solution is fed,some of the urea contained in the aqueous urea solution decomposes byheat in the exhaust gas and therefore ammonia is generated, so part ofthe NO_(x) in the exhaust gas is reduced by this ammonia. The amount ofthis ammonia, however, is also not that large, so the amount of NO_(x)in the exhaust gas reduced by this ammonia is not that large.

Therefore, when the temperature Tc of the catalyst 22 is maintained at aconstant temperature between about 132° C. and 350° C., even if feedingan aqueous urea solution so that the amount of urea becomes theequivalent ratio for the amount of NO_(x) in the exhaust gas, as shownin FIG. 2, the NO_(x) purification rate will not become that high. Inthis case, when the temperature of the exhaust gas becomes higher andthe temperature Tc of the catalyst 22 rises along with it, the amount ofammonia generated from the catalyst 22 will increase, while the amountof ammonia generated from the urea in the aqueous urea solution in theexhaust gas will increase as well. Therefore, as shown in FIG. 2, as thetemperature Ti of the exhaust gas flowing into the catalyst 22 rises,the NO_(x) purification rate also gradually rises.

When the temperature Tc of the catalyst 22 is maintained at asubstantially constant temperature between about 132° C. to 350° C., itis possible to increase the NO_(x) purification rate by increasing theamount of ammonia generated from the catalyst 22 and increasing theamount of ammonia generated from the urea in the aqueous urea solutionin the exhaust gas. Therefore, it is sufficient to increase the amountof urea fed. Therefore, in this embodiment of the present embodiment,when the temperature Tc of the catalyst 22 does not change that muchbetween about 132° C. and about 350° C., the amount of the aqueous ureasolution fed is increased so that the amount of urea fed becomes anamount of urea more than the equivalent ratio required for reducing theNO_(x) in the exhaust gas.

In this way, when the temperature Tc of the catalyst 22 does not changethat much between about 132° C. and about 350° C., it is possible toincrease the NO_(x) purification rate by feeding a urea in an amountmore than the equivalent ratio. If for example, as at the time ofacceleration, the temperature of the exhaust gas sharply rises and thetemperature Tc of the catalyst 22 sharply rises, even if an amount ofurea of more than the equivalent ratio is fed, the NO_(x) purificationrate cannot be increased.

That is, if shifting from a low load operation state to a high loadoperation state for acceleration, since the temperature of the exhaustgas sharply rises, the temperature Tc of the catalyst 22 also sharplyrises. On the other hand, if the engine operation state shifts to a highload operation, the amount of NO_(x) in the exhaust gas sharplyincreases. Even if however sharply increasing the amount of the aqueousurea solution fed so as to feed a urea in an amount more than theequivalent ratio required for reducing the NO_(x) sharply increasing atthis time, as explained above, ammonia is not generated from the sharplyincreased urea for a while. Experiments have found that almost noammonia is generated from the urea rapidly increased during accelerationoperation. Therefore, the amount of ammonia released from the catalyst22 at the time of acceleration operation becomes much less than theamount of ammonia required for reducing the increased NO_(x) andaccordingly a high NO_(x) purification rate cannot be obtained.

Therefore, in the present invention, in order to obtain a high NO_(x)purification rate when the amount of NO_(x) in the exhaust gas increasesand the temperature Tc of the catalyst 22 rises such as for example atthe time of acceleration operation, a large amount of urea, that is, alarge amount of the ammonia generating compound, is stored in thecatalyst 22 before the catalyst 22 starts to rise in temperature, alarge amount of ammonia is made to be released from the ammoniagenerating compound stored in the catalyst 22, and that large amount ofreleased ammonia is used to reduce the NO_(x) in the exhaust gas whenthe temperature Tc of the catalyst 22 sharply rises.

Explaining this in more detail, in the present invention, it is decidedif the temperature region of the catalyst 22 is in the ammoniagenerating compound storage region where the urea in the aqueous ureasolution, that is, the ammonia generating compound, is stored in thecatalyst 22 and almost no ammonia is released from the stored ammoniagenerating compound or in the ammonia release region where ammonia isreleased at a little at a time from the stored ammonia generatingcompound. In order to store a sufficient amount of ammonia generatingcompound in the catalyst 22 in advance for releasing the required amountof ammonia when the temperature of the catalyst 22 becomes the ammoniarelease region, a sufficient amount of aqueous urea solution required isfed into the catalyst 22 when it is decided that the temperature of thecatalyst 22 is in the ammonia generating compound storage region.

Here, the ammonia generating compound storage region indicates atemperature region where the temperature Tc of the catalyst 22 is lessthan about 132° C., while the ammonia release region indicates atemperature region where the temperature Tc of the catalyst 22 isbetween about 132° C. and about 350° C. When the temperature Tc of thecatalyst 22 is lower than about 132° C., as will be understood from FIG.4, the urea in the aqueous urea solution fed, that is, the ammoniagenerating compound, is stored in the catalyst 22. At this time, almostno ammonia is generated from the stored ammonia generating compound.Further, at this time, even if ammonia is generated from the urea in theexhaust gas, the amount is extremely small. Therefore, if an aqueousurea solution is fed when the temperature Tc of the catalyst 22 is inthe ammonia generating compound storage region, the majority of the ureain the aqueous urea solution, that is, the ammonia generating compound,is stored in the catalyst 22.

On the other hand, when the temperature Tc of the catalyst 22 is betweenabout 132° C. and about 350° C., that is, when the temperature Tc of thecatalyst 22 is in the ammonia release region, ammonia is released fromthe ammonia generating compound stored in the catalyst 22.

Briefly said, the temperature Tc of the catalyst 22 is in the ammoniagenerating compound storage region at the time of engine startup, enginewarmup, low load operation, and deceleration operation. Therefore, inthe present embodiment of the present invention, at the time of enginestartup, engine warmup, low load operation, and deceleration operation,a large amount of aqueous urea solution is fed so as to store urea, thatis, the ammonia generating compound, in the catalyst 22 to an extent notexceeding the maximum amount of the ammonia generating compound whichthe catalyst 22 can store. Therefore, at the time of accelerationoperation, a large amount of ammonia can be released from the ammoniagenerating compound stored in the catalyst 22 and therefore the NO_(x)in the exhaust gas can be purified well.

FIG. 5 shows an example of feed control of the aqueous urea solution.Note that FIG. 5 shows the changes in the required load L, the amount ofNO_(x) in the exhaust gas from the combustion chamber 5, the temperatureTc of the catalyst 22, the amount of feed of the aqueous urea solution,and the amount of ammonia generating compound stored in the catalyst 22.Note that in the amount of feed of the aqueous urea solution of FIG. 5,the broken line shows the amount of feed of the aqueous urea solution bywhich the amount of urea becomes the equivalent ratio for the NO_(x) inthe exhaust gas, while the solid line shows the amount of the aqueousurea solution actually fed.

In FIG. 5, the operation region I shows the time of engine startup orengine warmup or the time of low load operation including idling. Atthis time, as shown in FIG. 5, the amount of NO_(x) in the exhaust gasis small and the temperature Tc of the catalyst 22 is in the ammoniagenerating compound storage region. At this time, the aqueous ureasolution is fed so that the amount of feed of the urea becomes two tofour times the amount of urea of the equivalent ratio so that the amountof urea fed becomes more than the amount of urea of the equivalentratio. Therefore, at this time, the urea stored in the catalyst 22, thatis, the amount of storage of the ammonia generating compound, graduallyincreases.

Next, assume that the required load L has rapidly been raised and theengine is being accelerated. When the required load L is sharply raised,the amount of NO_(x) in the exhaust gas sharply increases. Further, atthis time, the temperature of the exhaust gas sharply rises, so thetemperature Tc of the catalyst 22 also sharply rises and the temperatureTc of the catalyst 22 enters the ammonia release region. At this time, alarge amount of ammonia is released from the ammonia generating compoundstored in the catalyst 22 and the NO_(x) in the exhaust gas is purifiedwell by the released ammonia. In this way, since a large amount ofammonia is released from the ammonia generating compound at this time,the amount of the ammonia generating compound stored in the catalyst 22sharply falls.

On the other hand, when all of the NO_(x) in the exhaust gas cannot bereduced by the ammonia generated from the ammonia generating compoundstored in the catalyst 22, aqueous urea solution is fed, even while theengine is being accelerated, to reduce the remaining NO_(x) in theexhaust gas by the ammonia generated from the urea in the aqueous ureasolution in the exhaust gas. In the example shown in FIG. 5, when theengine starts to be accelerated, the amount of feed of the aqueous ureasolution is reduced once, then increased. Of course, when the NO_(x) inthe exhaust gas can be sufficiently purified by the ammonia generatedfrom the ammonia generating compound stored in the catalyst 22 at thetime of acceleration operation, the feed of the aqueous urea solutionmay be stopped at the time of acceleration operation.

Next, assume that at the operation region II the engine is beingoperated steadily and that the temperature Tc of the catalyst 22 ismaintained at the ammonia release region at that time. At this time, inthe example shown in FIG. 5, the aqueous urea solution is fed so thatthe amount of feed of the urea becomes more than the amount of urea ofthe equivalent ratio so as to purify well the NO_(x) in the exhaust gas.Therefore, at this time, the amount of the ammonia generating compoundstored in the catalyst 22 is increased a little at a time.

Next, assume that at the operation region III the required load L isincreased, then the engine operated steadily under a high load and thatat the operation region III the temperature Tc of the catalyst 22exceeds about 350° C. and the temperature Tc of the catalyst 22 ismaintained at over about 350° C. under a high load operation state. Inthis case, if the temperature Tc of the catalyst 22 rises in theoperation region III, the amount of ammonia released from the ammoniagenerating compound stored in the catalyst 22 increases and thereforethe amount of ammonia generating compound stored in the catalyst 22decreases. Further, in the example shown in FIG. 5, the amount of feedof the aqueous urea solution is reduced at this time as well.

On the other hand, when the temperature Tc of the catalyst 22 exceedsabout 350° C., as explained above, all of the urea in the aqueous ureasolution fed is immediately decomposed by heat into ammonia and theNO_(x) in the exhaust gas is immediately reduced by this ammonia.Therefore, if making the amount of urea fed at this time the equivalentratio for the amount of NO_(x) in the exhaust gas, the NO_(x) in theexhaust gas can be completely purified. Therefore, as shown in FIG. 5,when the temperature Tc of the catalyst 22 becomes more than about 350°C., the aqueous urea solution is fed so that the amount of urea fedbecomes the equivalent ratio for the amount of NO_(x) in the exhaustgas. Note that at this time, no ammonia generating compound at all isstored in the catalyst 22. Therefore, at this time, as shown in FIG. 5,the amount of the ammonia generating compound stored in the catalyst 22becomes zero.

Next, assume that the engine is being decelerated and the supply of fuelis stopped. At this time, the amount of NO_(x) in the exhaust gasbecomes zero and the temperature Tc of the catalyst 22 sharply falls. Atthis time, in the example shown in FIG. 5, in the same way as theoperation region I, the aqueous urea solution is fed so that the amountof urea fed becomes more than the amount of urea of the equivalent ratiofor the amount of NO_(x) in the exhaust gas at the time of low loadoperation. Therefore, when the engine starts to be decelerated, theamount of ammonia generating compound stored in the catalyst 22 isincreased.

If more than the equivalent ratio of the urea is fed when thetemperature Tc of the catalyst 22 becomes more than about 350° C.,ammonia is discharged into the atmosphere. Therefore, when thetemperature Tc of the catalyst 22 becomes more than about 350° C., it isnecessary to control the amount of the aqueous urea solution fed toprecisely match the equivalent ratio.

On the other hand, even when the temperature Tc of the catalyst 22becomes less than about 350° C., if the amount of feed of the aqueousurea solution is increased by an extreme amount, ammonia is dischargedinto the atmosphere. Therefore, at this time as well, it is requiredthat the aqueous urea solution be fed so that no ammonia is dischargedinto the atmosphere. Note that experiments of the present inventors haveconfirmed that when changing the engine operation state by a fixedpattern and changing the temperature Tc of the catalyst 22 between about190° C. and about 350° C. at this time, when using a vanadium titaniacatalyst as the catalyst 22, no ammonia is discharged into theatmosphere even if feeding a urea in an amount close to three times theamount of urea of the equivalent ratio and that when using a copperzeolite catalyst as the catalyst 22, no ammonia is discharged into theatmosphere even if feeding a urea in an amount more than four times theamount of urea of the equivalent ratio.

The reason why the amount of ammonia discharged into the atmosphere issmaller when using a copper zeolite catalyst compared with when using avanadium titania catalyst is believed to be as follows. That is, if partof the ammonia generated from the ammonia generating compound is usedfor reduction of the NO_(x) in the exhaust gas on the surface of thecopper or vanadium and the remaining ammonia is held on the surface ofthe copper or vanadium, that ammonia becomes NO(NH₃→NO). Next, this NOreacts with the remaining ammonia to become N₂(NO+NH₃→N₂). If thissuccessive reaction occurs, the remaining ammonia will no longer bedischarged into the atmosphere.

Vanadium has a low ability to hold ammonia. Therefore, such a successivereaction does not easily occur and therefore ammonia is easilydischarged into the atmosphere. As opposed to this, copper has a highability to hold ammonia and therefore such a successive reaction easilyoccurs and ammonia is not easily discharged into the atmosphere.Accordingly, it is preferable to use a copper zeolite catalyst tosuppress the discharge of ammonia into the atmosphere.

Next, a first embodiment of the feed control of an aqueous urea solutionaccording to the present invention will be explained.

The amount of NO_(x) discharged from the combustion chamber 5 per unittime increases along with an increase in the engine load. Therefore, asshown in FIG. 6A, the amount of NO_(x) discharged from the combustionchamber 5 per unit time becomes greater the higher the temperature Ti ofthe exhaust gas flowing to the catalyst 22. Further, as shown in FIG.6B, the amount of NO_(x) discharged from the combustion chamber 5 perunit time is proportional to the amount Qa of intake air. Therefore, theamount QE of urea per unit time giving the equivalent ratio for theamount of NO_(x) in the exhaust gas becomes a function of thetemperature Ti of the exhaust gas and the amount Qa of the intake air.In this embodiment of the present invention, the amount QE of urea ofthe equivalent ratio to be fed per unit time is stored in advance in theROM 42 in the form of a map as shown in FIG. 6C as a function of thetemperature Ti of the exhaust gas and the amount Ga of the intake air.

Note that instead of calculating the amount QE of urea based on thetemperature Ti of the exhaust gas and the amount Qa of the intake air,it is also possible to detect the actual concentration of NO_(x) in theexhaust gas and calculate the amount QE of urea from the concentrationof NO_(x). In this case, an NO_(x) concentration sensor is attached inthe exhaust pipe 21 upstream of the catalyst 22, the amount of NO_(x)discharged from the combustion chamber 5 per unit time is found from theconcentration of NO_(x) detected by the NO_(x) concentration sensor andthe amount Ga of the intake air, and the amount QE of urea per unit timegiving the equivalent ratio for the amount of NO_(x) is calculated basedon this amount of NO_(x).

On the other hand, when the aqueous urea solution is fed, part of theurea contained in the aqueous urea solution is decomposed by heat in theexhaust gas and generates ammonia. In this case, the amount of ureadecomposed by heat increases the higher the temperature Ti of theexhaust gas flowing to the catalyst 22. Therefore, when the aqueous ureasolution is fed, the storage ratio ST of the urea stored in the catalyst22 falls the higher the temperature Ti of the exhaust gas as shown inFIG. 7.

Further, at the time of steady state operation when the temperature Tcof the catalyst 22 does not change that much, as explained above, theamount of ammonia released from the ammonia generating compound storedin the catalyst 22 increases along with the rise of the temperature Tcof the catalyst 22. Therefore, at this time, as shown in FIG. 8A, theratio NH₃ of ammonia released per unit time from the ammonia generatingcompound stored in the catalyst 22 increases the higher the temperatureTi of the exhaust gas flowing into the catalyst 22. On the other hand,the faster the space velocity of the exhaust gas, the greater the amountof ammonia released from the ammonia generating compound stored in thecatalyst 22. Therefore, as shown in FIG. 8B, the ratio NH₃ of ammoniareleased per unit time from the ammonia generating compound stored inthe catalyst 22 increases the greater the amount Ga of intake air. Inthis embodiment of the present invention, the ratio LE1 of ammoniareleased per unit time from the ammonia generating compound stored inthe catalyst 22 at the time of steady state operation is stored inadvance in the ROM 42 in the form of a map as shown in FIG. 8C as afunction of the temperature Ti of exhaust gas and the amount Ga ofintake air.

On the other hand, when the temperature of the catalyst 22 sharply risessuch as the time of acceleration operation, ammonia is released at alittle at a time from the ammonia generating compound stored in thecatalyst 22 before the rise of temperature of the catalyst 22 asexplained before. The ratio NH₃ of the ammonia released per unit timefrom the ammonia generating compound stored in the catalyst 22 at thistime changes in accordance with the temperature Ti of the exhaust gasflowing into the catalyst 22 as shown in FIG. 9A. Further, in this caseas well, as shown in FIG. 9B, the ratio NH₃ of ammonia released per unittime from the ammonia generating compound stored in the catalyst 22increases the greater the amount Ga of intake air. In this embodiment ofthe present invention, when the temperature Tc of the catalyst 22sharply rises, the ratio LE2 of the ammonia released per unit time fromthe ammonia generating compound stored in the catalyst 22 is stored inadvance in the ROM 42 in the form of a map as shown in FIG. 9C as afunction of the temperature Ti of the exhaust gas and the amount Ga ofthe intake air.

FIG. 10 to FIG. 12 show a routine for executing the first embodiment ofthe feed control of a aqueous urea solution. This routine is executed byinterruption every predetermined time period.

Referring to FIG. 10, first, at step 100, it is determined if the engineis starting up. When the engine is starting up, the routine jumps tostep 102, while when it is not starting up, the routine proceeds to step101. At step 101, it is determined if the engine is being decelerated.If the engine is being decelerated, the routine proceeds to step 102. Atstep 102, the EGR control valve 25 is made to close and the feed of theEGR gas is stopped. Next, the routine proceeds to step 103, where thefeed control I is executed, then the routine proceeds to step 104. Thisfeed control I is shown in FIG. 11.

On the other hand, when it is determined at step 101 that the engine isnot being decelerated, the routine proceeds to step 109, where it isdetermined if the temperature Ti of the exhaust gas detected by thetemperature sensor 35 is higher than a predetermined temperature, forexample, 350° C. When Ti is not more than 350° C., the routine proceedsto step 110, where it is determined if the engine is being accelerated.When it is not being accelerated, the routine proceeds to step 103. Thatis, the routine proceeds to step 103 when the engine is starting up andbeing decelerated and when Ti≦350° C. and the engine is not beingaccelerated.

Here, an explanation will be given of the feed control I performed atstep 103 while referring to FIG. 11.

Referring to FIG. 11, first, at step 200, it is determined if a feedstop flag showing that the feed of the aqueous urea solution should bestopped has been set or not. When the feed stop flag has not been set,the routine proceeds to step 201, where the amount QE of urea of theequivalent ratio to be fed per unit time is calculated from the mapshown in FIG. 6C based on the output signals of the air flow meter 14and the temperature sensor 35.

Next, at step 202, the ratio of the actual amount of urea fed to theequivalent ratio, that is, the ratio K of increase of urea, iscalculated. This ratio K of increase of urea, as shown in FIG. 13, islarger than 1.0. The ratio K of increase of urea becomes smaller as thetemperature Ti of the exhaust gas flowing to the catalyst 22 becomeshigher. In the example shown in FIG. 13, when the temperature Ti of theexhaust gas is low, the ratio K of increase of urea is made about 4.0.Next, at step 203, the ratio K of increase of urea is multiplied withthe amount QE of urea of the equivalent ratio to be fed per unit time soas to calculate the amount QE of urea (=K·QE) to be actually fed perunit time.

Next, at step 204, a correction coefficient C is multiplied with theamount QE of urea so as to calculate the amount Q of the aqueous ureasolution to be fed per unit time. When using an aqueous urea solution of30 wt % as the aqueous urea solution, the value of this correctioncoefficient C becomes (100+30)/30=4.3. When the amount Q of feed of theaqueous urea solution per unit time is calculated, the feed controlvalve 33 is controlled so that the amount of feed of the aqueous ureasolution becomes Q.

Next, at step 205, the storage ratio ST of urea is calculated from FIG.7. Next, at step 206, the amount QE of feed of urea is multiplied withthe storage ratio ST of urea so as to calculate the amount QST of urea(=QE·ST) stored in the catalyst 22 per unit time. Next, at step 207, theratio LE1 of release of ammonia is calculated from the map shown in FIG.8C. Next, at step 208, the ratio LE1 of release is multiplied with thetotal amount ΣQS of the ammonia generating compound stored in thecatalyst 22 so as to calculate the amount QLE of ammonia (=ΣQS·LE1)released per unit time. Next, the routine proceeds to step 104 of FIG.10.

On the other hand, when it is determined at step 200 that the feed stopflag has been set, the routine proceeds to step 209, where the amountQST of urea stored per unit time is made zero, then the routine proceedsto step 207. At this time, the feed of the aqueous urea solution isstopped. Therefore, when the engine is starting up or being deceleratedor when Ti≦350° C. and the engine is not being accelerated, the aqueousurea solution is fed so that the amount of feed of the urea becomes theamount of urea of the equivalent ratio so long as the feed stop flag isnot set.

At step 104 of FIG. 10, the total amount ΣQS of ammonia generatingcompound stored in the catalyst 22 is calculated based on the followingequation:ΣQS=ΣQS+QST−QLE.

Next, at step 105, it is determined if the amount ΣQS of the ammoniagenerating compound stored exceeds the maximum amount of storage MAX(FIG. 5). When ΣQS>MAX, the routine proceeds to step 108, where the feedstop flag is set. When the feed stop flag is set, the feed of theaqueous urea solution is stopped. On the other hand, when it isdetermined at step 105 that ΣQS≦MAX, the routine proceeds to step 106,where it is determined if ΣQS has become smaller than a certain valueMIN (<MAX). When ΣQS<MIN, the routine proceeds to step 107, where thefeed stop flag is reset.

On the other hand, when it is determined at step 110 that the engine isbeing accelerated, the routine proceeds to step 111, where the feedcontrol II is executed. This feed control II is shown in FIG. 12.

Referring to FIG. 12, first, at step 250, at the time of start ofacceleration operation, the total amount of ammonia generating compoundstored in the catalyst 22 is made the initial value ΣQS₀. Next, at step251, the ratio LE2 of release of ammonia is calculated from the mapshown in FIG. 9C. Next, at step 252, the ratio LE2 of release ismultiplied with the initial value ΣQS₀ of the total ammonia generatingcompound stored in the catalyst 22 to calculate the amount QLE ofammonia (=ΣQS₀·LE2) released per unit time.

Next, at step 253, the amount QE of urea necessary for reducing theNO_(x) in the exhaust gas which could not be reduced by the ammoniareleased from the ammonia generating compound in the catalyst 22 iscalculated. Next, at step 254, the correction coefficient C mentionedabove is multiplied with the amount QE of urea to calculate the amount Qof the aqueous urea solution to be fed per unit time. When the amount Qof feed of the aqueous urea solution per unit time is calculated, thefeed control valve 33 is controlled so that the amount of feed of theaqueous urea solution becomes Q.

Next, at step 255, the storage ratio ST of urea is calculated from FIG.7. Next, at step 256, the amount QE of feed of urea is multiplied withthe storage ratio ST of urea to calculate the amount QST of urea(=QE·ST) stored in the catalyst 22 per unit time. Next, the routineproceeds to step 104 of FIG. 10.

On the other hand, when it is determined at step 109 of FIG. 10 thatTi>350° C., the routine proceeds to step 112, where the amount QE ofurea of the equivalent ratio to be fed per unit time is calculated fromthe map shown in FIG. 6C based on the output signals of the air flowmeter 14 and the temperature sensor 35. Next, at step 113, thecorrection coefficient C mentioned above is multiplied with the amountQE of urea to calculate the amount Q of the aqueous urea solution to befed per unit time. When the amount Q of feed of aqueous urea solutionper unit time is calculated, the flow control valve 33 is controlled sothat the amount of feed of the aqueous urea solution becomes Q. Next, atstep 114, the amount ΣQS of storage of ammonia generating compound inthe catalyst 22 is made zero. In this way, when Ti>350° C., the aqueousurea solution is fed so that the amount of feed of urea becomes theamount of urea of the equivalent ratio.

Next, an explanation will be given of a second embodiment with referenceto FIG. 14 and FIG. 15.

When the amount of feed of the aqueous urea solution is small, theaqueous urea solution disperses in the exhaust gas. If the aqueous ureasolution disperses in the exhaust gas, the urea in the aqueous ureasolution will easily decompose by the heat and as a result it willbecome difficult to store the urea fed in the catalyst 22. As opposed tothis, if the amount of feed of the aqueous urea solution is increased,the density of the urea in the exhaust gas will become higher and as aresult the urea will become harder to decompose by heat, so the urea fedwill be able to be stored in the catalyst 22.

Therefore, in the second embodiment, as shown in FIG. 14, in theoperation region I, operation region II, and at the time ofdeceleration, the aqueous urea solution is fed so that the amount ofurea to be fed becomes the amount of urea of the equivalent ratio, theurea is used to reduce the NO_(x) in the exhaust gas, and large amountsof aqueous urea solution are fed in pulses at certain time intervals tomake the urea in the aqueous urea solution be stored in the catalyst 22.

Note that in the second embodiment as well, the routine shown in FIG. 10is used, but the routine shown in FIG. 15 is used for just step 103 ofFIG. 10.

Referring to FIG. 15, first at step 300, it is determined if the feedstop flag showing that the feed of aqueous urea solution should bestopped has been set. When the feed stop flag has not been set, theroutine proceeds to step 301, where the amount QE of urea of theequivalent ratio to be fed per unit time is calculated from the mapshown in FIG. 6C based on the output signals of the air flow meter 14and temperature sensor 35. Next, at step 302, it is determined if thetiming is the feed timing for feeding a large amount of aqueous ureasolution in a pulse in a short time. If the timing is the feed timing,the route proceeds to step 303, where it is determined if the feed timeof the aqueous urea solution has elapsed. When the feed time of theaqueous urea solution has not elapsed, the routine proceeds to step 304.

At step 304, the amount ΔQE of aqueous urea solution per unit time to befed in a pulse is calculated. The amount ΔQE of aqueous urea solution isdetermined so that the amount of urea fed becomes a predetermined amountof urea more than several times the amount of urea of the equivalentratio at the time of low load operation. Next, at step 306, theadditional amount ΔQE of urea is added to the amount QE of ureacalculated at step 301 to calculate the final amount QE of urea(=QE+ΔQE). Next, at step 307, the correction coefficient C mentionedabove is multiplied with the amount QE of urea to calculate the amount Qof the aqueous urea solution to be fed per unit time. When the amount Qof feed of the aqueous urea solution per unit time is calculated, theflow control valve 33 is controlled so that the amount of feed of theaqueous urea solution becomes Q.

Next, at step 308, the additional amount ΔQE of urea is made the amountQST of urea stored in the catalyst 22 per unit time. Next, at step 310,the ratio LE1 of release of ammonia is calculated from the map shown inFIG. 8C. Next, at step 311, the ratio LE1 of release is multiplied withthe total amount ΣQS of the ammonia generating compound stored in thecatalyst 22 to calculate the amount QLE of ammonia (=ΣQS·LE1) releasedper unit time. Next, the routine proceeds to step 104 of FIG. 10.

On the other hand, when it is determined at step 302 that the timing isnot the feed timing or when it is determined at step 303 that the feedtime has elapsed, the routine proceeds to step 305, where the additionalamount ΔQE of urea is made zero, then the routine proceeds to step 306.At this time, the amount of urea fed is made the equivalent ratio.

On the other hand, when it is determined at step 300 that the feed stopflag has been set, the routine proceeds to step 309, where the amountQST of urea stored per unit time is made zero, then the routine proceedsto step 310. At this time, the feed of the aqueous urea solution isstopped.

Next, an explanation will be given of a third embodiment while referringto FIG. 16 and FIG. 17.

When the temperature of the exhaust gas is low, the amount of NO_(x) inthe exhaust gas becomes extremely small. Therefore, in this thirdembodiment, when the temperature Ti of the exhaust gas flowing into thecatalyst 22 is lower than a predetermined value T₀, for example, 132°C., the continuous feed of the aqueous urea solution is stopped and, asshown in FIG. 16, in the operation region I and at the time ofdeceleration, large amounts of aqueous urea solution are fed in pulsesat time intervals to make the urea in the aqueous urea solution bestored in the catalyst 22.

Note that in the third embodiment as well, the routine shown in FIG. 10is used, but the routine shown in FIG. 17 is used only for step 103 ofFIG. 10.

Referring to FIG. 17, first, at step 400, it is determined if the feedstop flag indicating that the feed of the aqueous urea solution shouldbe stopped has been set. When the feed stop flag has not been set, theroutine proceeds to step 401, where it is determined if the temperatureTi of the exhaust gas flowing to the catalyst 22 is higher than apredetermined value T₀, for example, 132° C., from the output signal ofthe temperature sensor 35. When Ti>T₀, the routine proceeds to step 402,where the amount QE of urea of the equivalent ratio to be fed per unittime is calculated from the map shown in FIG. 6C based on the outputsignals of the air flow meter 14 and temperature sensor 35.

Next, at step 403, the ratio K of increase of urea is calculated fromFIG. 13. Next, at step 404, the ratio K of increase of urea ismultiplied with the amount QE of urea of the equivalent ratio to be fedper unit time so as to calculate the amount QE of urea (=K·QE) to beactually fed per unit time. Next, at step 405, the correctioncoefficient C mentioned above is multiplied with the amount QE of ureato calculate the amount Q of the aqueous urea solution to be fed perunit time. When the amount Q of feed of aqueous urea solution per unittime is calculated, the flow control valve 33 is controlled so that theamount of feed of the aqueous urea solution becomes Q.

Next, at step 406, the storage ratio ST of urea is calculated from FIG.7. Next, at step 407, the amount QE of feed of urea is multipled withthe storage ratio ST of urea to calculate the amount QST (=QE·ST) storedin the catalytic converter 22 per unit time. Next, at step 408, theratio LE1 of release of ammonia is calculated from the map shown in FIG.8C. Next, at step 409, the ratio LE1 of release is multiplied with thetotal amount ΣQS of the ammonia generating compound stored in thecatalyst 22 to calculate the amount QLE of ammonia (=ΣQS·LE1) releasedper unit time. Next, the routine proceeds to step 104 of FIG. 10.

On the other hand, when it is determined at step 401 that Ti≦T₀, theroutine proceeds to step 410, where it is determined if the timing is afeed timing for feeding a large amount of an aqueous urea solution in apulse in a short time. If the feed timing, the routine proceeds to step411, where it is determined if the feed time of the aqueous ureasolution has elapsed or not. When the feed time of the aqueous ureasolution has not elapsed, the routine proceeds to step 412.

At step 412, the amount QEE of urea per unit time to be fed in a pulseis calculated. This amount QEE of urea is determined to give apredetermined amount of urea more than several times the amount of ureaof the equivalent ratio. Next, at step 413, the correction coefficient Cmentioned above is multiplied with the amount QEE of urea to calculatethe amount Q of the aqueous urea solution to be fed per unit time. Whenthe amount Q of feed of the aqueous urea solution per unit time iscalculated, the flow control valve 33 is controlled so that the amountof feed of the aqueous urea solution becomes Q. Next, at step 414, QEEis made the amount QST of urea stored in the catalyst 22 per unit time.Next, the routine proceeds to step 408.

On the other hand, when it is determined at step 400 that the feed stopflag has been set, the routine proceeds to step 415, where the amountQST of urea stored per unit time is made zero, then the routine proceedsto step 408. At this time, the feed of the aqueous urea solution isstopped.

Note that in the second embodiment and the third embodiment, it ispossible to change the amount and timing of feed of the pulse-like fedaqueous urea solution. For example, it is also possible to reduce theamount of feed of the aqueous urea solution at the time of decelerationa little at a time with each pulse-like feed. Further, to wait for thetemperature Tc of the catalyst 22 to sufficiently fall, it is alsopossible to extend the interval until the aqueous urea solution is fedafter the start of deceleration the higher the temperature Tc of thecatalyst 22 at the time of start of deceleration.

Next, an explanation will be given of various examples of the catalyst22 held in the catalytic converter 23 while referring to FIG. 18 andFIG. 19.

As shown in FIG. 18A, the catalyst 22 has a honeycomb structure and isprovided with a large number of exhaust gas passages 61 surrounded bythe honeycomb structure substrate 60. A catalyst layer is formed on thesurface of the substrate 60 forming the exhaust gas passages 61. In theexample shown in FIG. 18B, the catalyst layer 62 is comprised of titania63. Vanadium 64 is carried on this titania 63. When the aqueous ureasolution is fed, the urea, that is, the ammonia generating compound,contained in the aqueous urea solution is stored in the carrier titania63. It is not necessarily clear how the ammonia generating compound isstored in the titania 63, but probably it is held in the catalyst 22 byadsorption.

In the present invention, the ammonia released a little at a time fromthe urea, that is, the ammonia generating compound, stored in thecatalytic converter 22 is used to reduce the NO_(x) in the exhaust gas,so it can be said to be preferable to hold as large an amount of urea,that is, the ammonia generating compound as possible, in the catalyst 22when the aqueous urea solution is fed and to cause the ammonia to bereleased a little at a time from the ammonia generating compound. FIGS.18C, 18D, and 18E and FIGS. 19A and 19B show examples where as large anamount of ammonia generating compound as possible is held in thecatalyst 22.

That is, in the example shown in FIG. 18C, a zeolite layer 65 is formedon a carrier comprised of titania 63. By doing this, the urea, that is,the ammonia generating compound, is held in the zeolite layer 65 aswell, so the amount of the ammonia generating compound held isincreased. The ammonia generating compound held in the zeolite layer 65disperses into the carrier comprised of the titania 63, then isdecomposed by heat, so ammonia is slowly released.

In the example shown in FIG. 18D, a zeolite layer 65 is formed between asubstrate 60 and a carrier comprised of titania 63. In this case aswell, the urea, that is, the ammonia generating compound, is held in thezeolite layer 65 as well, so the amount of the ammonia generatingcompound held is increased. The ammonia generating compound held in thezeolite layer 65 disperses into the carrier comprised of the titania 64,then is decomposed by heat, so ammonia is slowly released.

In the example shown in FIG. 18E, the catalyst layer 62 is comprised oftitania and zeolite. Vanadium is carried on this carrier comprised oftitania and zeolite.

In the example shown in FIGS. 19A and 19B, in the region X at theupstream side of the catalyst 22, the catalyst layer 62 is comprised ofzeolite, while in the region Y at the downstream side of the catalyst22, the catalyst layer 62 is comprised of titania 63 carrying vanadium64. In this example as well, the urea, that is, the ammonia generatingcompound, is held in the zeolite layer 65 as well, so the amount of theammonia generating compound held is increased. The ammonia generatingcompound held in the zeolite layer 65 disperses into the carriercomprised of the titania 63, then is decomposed by heat, so ammonia isslowly released.

FIG. 20 to FIG. 23 show other embodiments of an internal combustionengine. In the example shown in FIG. 20, a further catalytic converter70 is arranged downstream from the catalytic converter 23. As thecatalyst 22 housed in the catalytic converter 70, it is possible to useeither of a copper zeolite catalyst or a vanadium titania catalyst inthe same way as the catalyst 22 housed in the catalytic converter 23.However, when using both of these catalysts 22, it is preferable to makethe upstream side catalyst 22 in the catalytic converter 23 a vanadiumtitania catalyst and make the downstream side catalyst 22 in thecatalytic converter 70 a copper zeolite catalyst. This is because theammonia flowing out from the vanadium titania catalyst can be removed onthe copper zeolite catalyst.

In the embodiment shown in FIG. 21, a pair of catalysts 22 a and 22 bare arranged at a distance from each other in the catalytic converter23. Further, in this embodiment, the exhaust pipe 21 is branched into afirst exhaust passage 71 a opening to the catalytic converter 23 and asecond exhaust passage 71 b connected to a space between the catalysts22 a and 22 b. A first exhaust control valve 72 a and a second exhaustcontrol valve 72 b are arranged in the exhaust passages 71 a and 71 b,respectively. Further, in this embodiment, the aqueous urea solution isfed into the exhaust pipe 21 upstream of the two exhaust control valves72 a and 72 b.

In this embodiment, when the temperature of the exhaust gas detected bythe temperature sensor 35 is lower than about 150° C., as shown in FIG.21, the first exhaust control valve 72 a is made to fully open and thesecond exhaust control valve 72 b is made to fully close. At this time,the exhaust gas first passes through the upstream side catalyst 22 a,then passes through the downstream side catalyst 22 b. At this time, thetemperature of the catalysts 22 a and 22 b is in the ammonia generatingcompound storage region, therefore the majority of the urea in theaqueous urea solution fed is stored in the upstream side catalyst 22 a.

On the other hand, while the temperature of the exhaust gas detected bythe temperature sensor 35 is between about 150° C. and 250° C., thefirst exhaust control valve 72 a is made to fully close and the secondexhaust control valve 72 b is made to fully open. Therefore, at thistime, the exhaust gas passes through the second exhaust passage 71 b andthen passes through the downstream side catalyst 22 b. At this time, theurea stored in the upstream side catalyst 22 a is held as it is, and theNO_(x) of the exhaust gas is purified at the downstream side catalyst 22b by the aqueous urea solution fed.

On the other hand, when the temperature of the exhaust gas detected bythe temperature sensor 35 becomes more than about 250° C., the firstexhaust control valve 72 a is made to fully open again and the secondexhaust control valve 72 b is made to fully close again. When the engineis being accelerated, the exhaust gas detected by the temperature sensor35 becomes more than 250° C., so when the engine is being accelerated,the exhaust gas flows into the upstream side catalyst 22 a. At thistime, ammonia is released a little at a time from the large amount ofammonia generating compound stored in the upstream side catalyst 22 a,and the NO_(x) in the exhaust gas is reduced in the upstream sidecatalyst 22 a and the downstream side catalyst 22 b by this ammonia.

In the embodiment shown in FIG. 22, unlike the embodiment shown in FIG.21, an aqueous urea solution is fed in the first exhaust passage 71 adownstream of the first exhaust control valve 72 a.

In this embodiment, when the required load of the engine is lower than apredetermined set load, the first exhaust control valve 72 a is made tofully close and the second exhaust control valve 72 b is made to fullyopen. Therefore, the exhaust gas passes through the second exhaustpassage 71 b and then passes through the downstream side catalyst 22 b.Further, at this time, the exhaust gas does not flow through the firstexhaust passage 71 a and the temperature of the upstream side catalyst22 b is in the ammonia generating compound storage region. Therefore, atthis time, the majority of the urea in the aqueous urea solution fed isstored in the upstream side catalyst 22 a.

On the other hand, when the required load of the engine becomes higherthan the set load, as shown in FIG. 22, the first exhaust control valve72 a is made to fully open and the second exhaust control valve 72 b ismade to fully close. At this time, the exhaust gas first passes throughthe upstream side catalyst 22 a, then passes through the downstream sidecatalyst 22 b. Therefore, ammonia is released at little at a time fromthe large amount of the ammonia generating compound stored in theupstream catalyst 22 a, and the NO_(x) in the exhaust gas is reduced inthe upstream side catalyst 22 a and the downstream side catalyst 22 b bythis ammonia.

In the embodiment shown in FIG. 23, a ring-shaped exhaust passage 74intersecting with the exhaust pipe 21 at a flow path switching valve 73,arranged in the exhaust pipe 21, is provided. A first catalyticconverter 23 and a second catalytic converter 70 are arranged in seriesin the ring-shaped exhaust passage 74.

In this embodiment, when the required load of the engine is lower than apredetermined set load, the flow path control valve 73 is switched to aposition shown by the solid line in FIG. 23 so that the exhaust gaswhich had been sent through the exhaust pipe 21 flows in the directionof the arrow A, that is, first passes through the catalyst 22 in thefirst catalytic converter 23, then passes through the catalyst 22 in thesecond catalytic converter 70. At this time, the aqueous urea solutionis fed to the upstream side of the second catalytic converter 70. Atthis time, the temperature of the catalyst 22 in the second catalyticconverter 70 is lower than the temperature of the catalyst 22 in thefirst catalytic converter 23 and therefore the urea, that is, theammonia generating compound, in the aqueous urea solution fed is storedwell in the catalyst 22 in the second catalytic converter 70.

On the other hand, when the required load of the engine becomes higherthan the set load, the flow path control valve 73 is switched to theposition shown by the broken line in FIG. 23 so that the exhaust gaswhich had been sent through the exhaust pipe 21 flows in the directionof the arrow B, that is, first passes through the catalyst 22 in thesecond catalytic converter 70, then passes through the catalyst 22 inthe first catalytic converter 23. At this time, the temperature of thecatalyst 22 in the second catalytic converter 70 becomes higher than thetemperature of the catalyst 22 in the first catalytic converter 23 andtherefore ammonia is released well from the ammonia generating compoundstored in the catalyst 22 in the second catalytic converter 70.

Up to there, the present invention had been explained taking as anexample the case of use of an aqueous urea solution as the liquidcontaining an ammonia generating compound. In this case, as mentionedabove, it is also possible to use something other than urea as theammonia generating compound and possible to use something other thanwater as the solvent. Further, it is also possible to feed ammonia wateror a gas containing ammonia together with the liquid containing anammonia generating compound into the exhaust passage. In this case, thegas containing ammonia may be generated using solid urea.

1. An exhaust gas purification device of an internal combustion enginearranging a catalyst suitable for reduction of NO_(x) in exhaust gas byammonia in the presence of excess oxygen in an engine exhaust passage,said device comprising: feeding means for feeding a liquid containing anammonia generating compound to the catalyst; feed control means forcontrolling the amount of feed of the liquid, the catalyst storinginside it at least part of the ammonia generating compound contained inthe liquid fed to the catalyst and releasing ammonia from the ammoniagenerating compound stored in the catalyst to reduce the NO_(x) in theexhaust gas by the ammonia released; wherein, when the catalyst is in astate where ammonia can be released from the catalyst, and said feedingmeans feeds the liquid, said feed control means controls the amount offeed of the liquid so that the amount of the ammonia generating compoundfed is made more than the equivalent ratio=1 even when the temperatureof the catalyst is maintained at a substantially constant temperatureuntil the amount of storage of the ammonia generating compound exceeds apredetermined set level.
 2. An exhaust gas purification device of aninternal combustion engine as set forth in claim 1, wherein the feed ofthe liquid is stopped when the amount of storage of the ammoniagenerating compound exceeds the predetermined set level.
 3. An exhaustgas purification device of an internal combustion engine as set forth inclaim 1, wherein the liquid containing the ammonia generating compoundis an aqueous urea solution.